Many gas turbine engine fuel supply systems include a fuel source, such as a fuel tank, and one or more pumps that draw fuel from the fuel source and deliver pressurized fuel to the fuel manifolds in the engine combustor via a main supply line. The main supply line may include one or more valves in flow series between the pumps and the fuel manifolds. These valves generally include, for example, a main metering valve and a pressurizing-and-shutoff valve downstream of the main metering valve. In addition to the main supply line, many fuel supply systems also include a bypass flow line connected upstream of the metering valve that bypasses a portion of the fuel flowing in the main supply line back to the inlet of the one or more pumps, via a bypass valve. The position of the bypass valve is typically controlled by a head regulation scheme to maintain a substantially fixed differential pressure across the main metering valve.
The above-described fuel supply system is generally safe, reliable, and robust. Nonetheless, it can suffer certain drawbacks. For example, the metering valve and bypass valve can add to overall system weight and complexity. Moreover, control of the metering valve and bypass valve can result in increased system complexity and cost. Thus, in recent years there has been a desire to implement more direct metering fuel control systems. In such systems fuel flow rate is controlled by controlling, for example, the speed or the displacement of the fuel metering pump. Yet, efforts to implement direct fuel metering control systems have also been impeded by certain drawbacks. For example, many gas turbine engines include one or more fluid-operated actuators. In many instances these actuators are driven by the fuel from the fuel supply system. Thus, actuator operation can cause a droop in the fuel supplied to the engine, and thus an undesirable engine speed droop. It is also postulated that such fuel supply variations to the engine could yield unpredictable, and potentially less controllable, engine transients. Moreover, many current direct metering fuel systems are not as thermally efficient and cost effective as desired, and some systems may not exhibit sufficient redundancy and/or reliability.
Hence, there is a need for a system and method of controlling fuel flow in a direct metering fuel control system and does not cause, or that at least substantially lessens, fuel supply variations to the engine and/or does not yield unpredictable and/or relatively less controllable engine transients and/or exhibits increase thermal efficiency and cost effectiveness, and/or exhibits sufficient redundancy and/or reliability. The present invention addresses one or more of these needs.